The aim of the proposed research is to define the role of Brain-Derived Neurotrophic Factor (BDNF) in activity-dependent plasticity in the developing chemoafferent pathway. Chemoafferent neurons are the link between peripheral chemoreceptors and the brainstem, and thereby play a pivotal role in cardiorespiratory homeostasis. At birth, chemoafferent reflexes are immature, and perturbations in oxygen availability can derange postnatal development of cardiorespiratory responses to acute hypoxia. However, mechanisms that underlie chemoreflex development and plasticity are largely undefined. This proposal is based on our recent discoveries that 1) Chemoafferent neurons in the newborn rat nodose-petrosal ganglion complex (NPG) express high levels of BDNF messenger RNA and protein, 2) BDNF protein is released from NPG neurons in response to patterned electrical stimulation in vitro, and 3) BDNF acutely inhibits glutamatergic AMPA receptors in second-order relay neurons in the nucleus tractus solitarius (nTS), the primary site of chemoafferent projections to the brainstem. Together, these data indicate a new role for BDNF as a modulator of excitatory synaptic transmission between primary chemoafferent neurons and second-order relay neurons in nTS. In view of increasing evidence that BDNF plays a critical role in long-term synaptic plasticity elsewhere in the brain, we hypothesize that BDNF plays a similar role at chemoafferent synapses in nTS. Moreover, based on our preliminary data, we hypothesize that BDNF signaling in nTS is regulated by changes in oxygen availability, and thereby contributes to derangements in chemoreflex function following chronic sustained or intermittent hypoxia. Therefore, the proposed research is designed to further define mechanisms of BDNF expression and release in chemoafferent neurons after birth, including the role of chronic sustained and intermittent hypoxia, in vivo and in vitro. In addition, we will characterize postsynaptic effects of BDNF on developing nTS neurons, including regulation of transmitter receptor expression and dendritic growth. Moreover, we will determine the role of BDNF in functional plasticity in vivo by analyzing development of peripheral chemoreflexes in transgenic mice in which BDNF signaling is disrupted selectively after birth. By defining mechanisms of activity-dependent plasticity in the PG and nTS, the proposed research may shed light on cellular and molecular mechanisms relevant to understanding and improved management of hypoventilation and apnea syndromes in neonates and infants, as well as mechanisms that contribute to altered cardiorespiratory control in adult obstructive sleep apnea and chronic obstructive pulmonary disease. Moreover, it is hoped that elucidating development of this system will, in turn, create a model of neurotrophin function applicable to the nervous system as a whole.